This invention relates to an ultrasonic Doppler imaging apparatus for use in ultrasonic diagnosis in the medical diagnosis so as to make real time measurement of a velocity of blood flow in a living body.
An ultrasonic Doppler imaging apparatus for the medical diagnosis is already known, in which the technique of ultrasonic pulse Doppler measurement and the technique of pulse reflection are used in combination so as to obtain both information of a blood flow and information of a tomographic image (a B-mode image) in a living body by the use of an ultrasonic probe and then to display a real time color image of the blood flow information superposed on the tomographic image. An ultrasonic diagnostic apparatus disclosed in JP-A-63-59938 is an example of such an ultrasonic Doppler imaging apparatus.
A conventional ultrasonic Doppler imaging apparatus will now be described hereunder before describing the present invention.
FIG. 1 shows the structure of a conventional ultrasonic Doppler imaging apparatus. Referring to FIG. 1, ultrasonic waves reflected from a body under test is received by an ultrasonic probe 271 and is then amplified through a receiving circuit 272. This received signal includes both a wave component reflected from a stationary part of the body under test and a wave component reflected from a moving part of the body under test. An output signal of the receiving circuit 272 is supplied in parallel with a first and a second signal processing system.
In the first signal processing system, the output signal of the receiving circuit 272 is amplified and detected through a detecting circuit 273 to provide a B-mode signal, and then this B-mode signal is converted in a DSC (digital scan converter) 274 to a digital signal, which is stored in an image memory 274a contained in the DSC 274. A clutter decision level is set beforehand in a clutter map 275, and the level of each signal stored in the image memory 274a is compared with the clutter decision level. When the level of each of the stored signals exceeds the clutter decision level, an address of that signal is recorded, so that a map showing positions, where the clutters exist, is prepared.
In the second signal processing system, the output signal of the receiving circuit 272 is applied to each of two MTI (moving target indicator) processing circuits 277 and 278. One of the two MTI processing circuits, that is, the low-speed MTI processing circuit 277, generates its output signal without eliminating reflected waves coming from a slowly moving target. Therefore, when the clutter signals exist in an input signal to the low-speed MTI processing circuit 277, an output signal from the low-speed MTI processing circuit 277 is generated without sufficiently eliminating the clutter signals.
The other high-speed MTI processing circuit 278 generates an output signal, in which reflected waves coming from a moving target, whose position does not change appreciably, are eliminated, so that the clutter signals contained therein are also completely eliminated. Therefore, where the clutter signal does not exist, reflected waves coming from a slowly moving target are eliminated. These two MTI processing circuits 277 and 278 are switched over by a switch 279 controlled by a switch control circuit 276. An output signal of the receiving circuit 272 is usually processed by the low-speed MTI processing circuit 277 to display a color image of a moving target. However, when a sound ray is incident on a position where the clutter signal exists, that is, when a clutter address signal in the clutter map 275 coincides with a digitized B-mode signal stored at an address of the image memory 274a in the DSC 274, an output signal of the clutter map 275 is applied to the switch control circuit 276, so that the switch 279 is changed over to the side of the high-speed MTI processing circuit 278. Then, the output signal of the receiving circuit 272 is processed by the high-speed MTI processing circuit 278, and the output signal of this MTI processing circuit 278 is applied to the DSC 280.
In the manner described above, the existing positions of the clutter signals are recorded on the clutter map 275. Whenever the existence of the clutter signal is detected, the switch 279 is changed over to successively store the output signal of the high-speed MTI processing circuit 278 in an image memory 280a in the DSC 280. Then, the contents of the image memory 280a are read out and supplied to a color image synthesizing circuit 281 to be synthesized into a color image, and the synthesized color image is displayed on a display section 282.
However, the conventional ultrasonic Doppler imaging apparatus has a drawback such that it is necessary for this apparatus to comprise the two MTI processing circuits, that is, the low-speed MTI processing circuit 277 and the high-speed MTI processing circuit 278, thereby resulting in an undesirable increase in the scale of the signal processing circuit.